Various attempts to fabricate practical and economical thermophotovoltaic (TPV) power generators have been reported over the years. In 1986, D.O.Pelka, A. Santos and W.W. Yuen described earlier efforts and their attempt to design and fabricate a natural gas-fired thermophotovoltaic system. Their desk top sized unit used a rotating drum with a porous particle bed serving as the high temperature emitter operating at &gt;2000.degree. K. Silicon TPV cells were mounted inside the rotating drum. They noted that for silicon cells and an emitter temperature &lt;2600.degree. K., over 76% of the black body radiation is in unusable long wavelength (&gt;1.1 microns) energy. Their design was complicated by their attempt to design a very high temperature emitter as required by the silicon TPV cells. Without very high temperature emitters, TPV systems based on silicon cells are both inefficient and operate at low power densities. Selective emitters based on rare earth oxides have been described (see U.S. Pat. No. 4,976,606) which improve efficiencies but still suffer from low power densities at practical emitter temperature. Low power density units are not economical for large volume energy production.
In 1989, L.M.Fraas et. al. described a new GaSb photovoltaic cell sensitive in the IR out to 1.8 microns. This cell was designed to be used with concentrated sunlight as an infrared booster cell in tandem with GaAs solar cells (see U.S. Pat. No. 5,096,505). In 1989, M.D.Morgan, W.E.Horne, and A.C.Day (NASA SPRAT conference) proposed using GaSb cells in combination with a radioisotope thermal generator for space electric power and in 1991, O.L.Doellner proposed using GaSb cells looking at jet engine plumes to replace alternators on jet aircraft. As of this writing (April 1993), neither Morgan nor Doellner have built a TPV generator using GaSb cells.
It now seems timely to take a fresh approach to the design of a compact natural gas-fired TPV generator. It is clear that the longer wavelength response of the GaSb cell will allow the use of lower temperature emitters. However, several problems must still be solved. First, when photovoltaic cells are wired together in series in order to generate a required voltage, eg. 12 V, it is important that they each receive the same amount of IR radiation. Otherwise, the cell string current is limited by the cell with the lowest IR generated current. This translates to a requirement to tailor the temperature profile over a large region of the emitter surface. Second, energy conversion efficiency is not only controlled by the TPV cell bandgap and response to the IR; exhaust gas heat losses up the stack can be appreciable without providing for regenerative heating of the supply air by the exhaust gas. This will also increase the flame temperature and permit higher emittter temperatures, which will in turn increase the cell output per unit of cell area, and thus reduce the size, weight, and cost. Third, the low bandgap TPV cell temperature must be maintained near room temperature in order to preserve high cell conversion efficiency. Forth, the IR energy from the emitter has to be efficiently coupled to the TPV cell strings. And finally, fifth, it may be desirable to tune the low bandgap response even somewhat further into the infrared.